According to tightening regulations on automobile emissions, there has been a demand for maintaining exhaust gas purification performance in an exhaust gas treatment device over a long time. This means that a demand for long service life of a catalyst as an aftertreatment device for exhaust gas purification has increased and also means that there has been a demand for improving long-term durability of a catalyst. In order to improve long-term durability of a catalyst, suppression of sintering of noble metal particles supported on a catalyst or suppression of sulfur poisoning has been studied. On the other hand, it is known that poisoning due to phosphorus contained in an exhaust gas (hereinafter, referred to as phosphorus poisoning) exerts a great influence on deterioration of catalytic performance (Non-Patent Literature 1). In order to meet regulations that will be tightened more and more from now, it is important to suppress the phosphorus poisoning or to maintain and improve the catalytic performance after phosphorus poisoning.
The phosphorus poisoning is known to be caused by accumulating or infiltrating, into a catalyst layer, a phosphorus compound derived from a lubricant additive, such as zinc dialkyl dithiophosphate, which is contained in an exhaust gas. It is known that occurrence of the phosphorus poisoning leads to deterioration of catalytic performance (Non-Patent Literature 1).
The catalytic performance deterioration due to the phosphorus poisoning is known to be caused by occurrence of phenomena as described below. Diffusional inhibition of the exhaust gas in the catalyst layer is caused by the phosphorus compound accumulated or infiltrated into the catalyst layer. In addition, cerium oxide of an oxygen-storage-release material that is widely used in a three-way catalyst and a phosphorus compound react to each other to form cerium phosphate. When cerium phosphate is formed, oxygen is difficult to be stored or released. Therefore, when atmosphere of the exhaust gas is fluctuated to be lean or rich, atmosphere fluctuation is less likely to be alleviated. According to occurrence of these phenomena, exhaust gas purification efficiency is deteriorated.
It is disclosed that, in a catalyst particularly using palladium in order to suppress phosphorus poisoning, performance deterioration can be alleviated by using a complex oxide of ceria and zirconia as an oxygen-storage-release material (Patent Literature 1).
Further, it is disclosed that performance deterioration due to phosphorus poisoning is suppressed by providing a region which is not washcoated as a phosphorus capture zone at a front end of a catalyst structure (Patent Literature 2).